This invention relates to alkali resistant, synthetic, mineral fibres, and more particularly to mineral fibres useful for the reinforcement of cementitious products.
Cementitious products, such as products made of concrete based on Portland cement or other types of cement or mixtures of cement and other types of binders, exhibit high compression strengths but low tensile strengths.
It is well known that the tensile strengths of cementitious products can be increased by incorporating therein a fibrous reinforcement. Thus, asbestos fibres have found widespread use as a reinforcing material in cement products for use in the building industry.
Unfortunately, the use of asbestos fibres presents serious health problems and, therefore, the use of asbestos fibres has been prohibited in some countries and is likely to be prohibited in other countries.
Many attempts have been made to replace asbestos fibres with synthetic mineral fibres, such as rock wool fibres, slag fibres and glass fibres in fibre reinforced cementitious products.
However, cementitious products reinforced by synthetic mineral fibres do not exhibit fully satisfactory strength properties because the mineral fibres do not have a sufficient resistance to attack by the alkaline components of the cement matrix to prevent the fibres from being partially or wholly decomposed.
According to the accepted German patent application No. 1.796.213 it has been attempted to increase the alkali resistance of glass fibres by coating the fibres with a methyl siloxane and by manufacturing the glass fibres from such starting materials that the fibres produced have a composition falling within the following ranges:
______________________________________ SiO.sub.2 : 35-47% by weight Al.sub.2 O.sub.3 : 5-18% by weight FeO+Fe.sub.2 O.sub.3 : 2-15% by weight CaO: 2-23% by weight MgO: 1-30% by weight Na.sub.2 O+K.sub.2 O: up to 4% by weight TiO.sub.2 : up to 10% by weight and either ZrO.sub.2 : up to 10% by weight or ZnO: up to 5% by weight ______________________________________
the sum of CaO and MgO being from 14 to 38% by weight.